Proportional fonts are fonts whose characters take up more or less space depending on the individual character. For example, an "i" will take less space than an "o" will. Proportional fonts are also called variable width fonts. Common examples include Times and Helvetica. Fixed width fonts are fonts whose characters all take up the same amount of space.

Fonts that have letters or characters of varying widths. The horizontal space occupied by each characters is "propotional" to its shape, therefore an "l" takes up less space than a "w". This is why it is impossible to line up vertical data using just the space bar if you are using proprtional fonts. This is also why the unit of measurment for size is a vertical measurment, points.